The potential demand for wireless communications combined with restricted availability of the radio frequency spectrum has motivated intense research into bandwidth efficient multiple access schemes. One approach resulting from this pursuit has been the use of direct sequence spread spectrum-code division multiple access (i.e. DS/SS-CDMA) communication. Such a technique takes advantage of available bandwidth on the applicable transmission medium by generating a set of pulses in the time domain which have appropriate correlation properties over predetermined time periods.
Direct sequence spread spectrum (DS/SS) radio transmission systems, in contrast to more traditional radio transmission systems, use a signal bandwidth that is much broader than the information signal bandwidth. A wide band signal is generated by multiplying the narrowband information signal with a binary code, often designated as a spreading or pseudo-noise (PN) code, to generate the wideband signal that is transmitted. The original information signal can be re-created at the receiver by multiplying the received wideband signal by the same binary code (now designated as a de-spreading or pseudo-noise code) used to generate the wideband transmitted signal. In order to recover the intelligence the spreading and de-spreading codes must be in synchronism and have matching amplitudes.
Direct sequence spread spectrum transmission technology is now being applied to multi-user transmission systems such as cellular radio telephone systems. In such applications it is designated as code division multiple access (CDMA) to distinguish it from the prior time division multiple access (TDMA) and frequency division multiple access (FDMA) systems now in use. In the CDMA system the individual user channels (which are not distinguished by time of transmission or frequency differences) are each individually identified by a unique spreading and de-spreading code at both the transmitting and receiving end which is used to recover the individual user's signal from the signals of other users and from background noise and interference.
One system that has recognized the attendant advantages of the aforementioned technique is disclosed in U.S. Pat. No. 5,150,377 entitled Direct Sequence Spread Spectrum (DSSS) Communications System With Frequency Modulation Utilized To Achieve Spectral Spreading issued to Vannucci on 22 Sep. 1992. In this invention, the spectral spreading technique of a code-division multiple access (CDMA) system is generalized by extending the range of values allotted to the spreading waveform code signal to include complex numbers of unity magnitude. While this invention may be useful in its own right, we believe it can be improved upon in order to reduce the amount of interference that results when propagation distances between the respective mobile stations are different.
Another system taking advantage of the favorable characteristics of spread spectrum communication is disclosed in U.S. Pat. No. 5,175,744 entitled Spread-Time Code Division Multiple Access Technique With Arbitrary Spectral Shaping issued to Yerbury et al. on 5 Nov. 1991. In this system, pseudo-random noise (PN) sequences are assigned to each transmitter and the Fourier transform of the transmitter pulse is determined by modulating the phase of the desired transmitter spectrum by the pseudo-random (PN) sequence assigned to the transmitter. This invention, like Vanucci '377, contains merit in its own respect. We believe, however, that interference between incoming signals can be reduced further by using our disclosed techniques.
One early effort that specifically addresses the removal of interference between incoming signals (e.g. when the incoming signals propagate along paths of unequal distance) is U.S. Pat. No. 5,099,493 entitled "Multiple Signal Receiver For Direct Sequence, Code Division Multiple Access, Spread Spectrum Signals" issued on 24 May 1992 to Zegar et al. This reference teaches a method for enabling the level of signals received from a plurality of mobile stations to be constant. This is accomplished by identifying the signal outputs received from the plurality of mobile stations, and selectively amplifying and attenuating the received signals to thereby overcome the problems associated with a "near-far" effect, caused by the aforementioned circumstances of incoming signals propagating along paths of unequal distance.
In this prior art method, however, it is very difficult to adjust the high-frequency outputs of the plurality of mobile stations so that their respective output levels are identical to each other. Moreover, many difficulties are associated with a product capable of performing this method since a high degree of accuracy is required for executing this type of adjustment.